What does ISIS say about the Rashidun Caliphate? I can't say that I've read much about it. I can make some general observations though. Even though the rise of Islam was the defining event of the seventh century, it is important to stress that we know very little about it due to the lack of contemporary sources. I've cobbled together comments on the earliest sources for Muhammad and the reasons for the success of the conquests, but it is nonetheless clear that there is so much more that we don't know about this period. Therefore all reconstructions of the past are all inevitably incorrect in parts, so do keep this in mind when you read about this fascinating period.

I'm assuming ISIS primarily uses Muslim sources to reconstruct their past (which is perfectly legitimate, since although they were written much later, there must be a lot of accurate information from the ninth-century narratives, historians just have to use them with care), but even with these later harmonised sources it is very easy to argue that their past wasn't so glorious. Leaving aside the problem of defining "glorious" (was it a purely military thing, do we see it in cultural terms etc), it is clear that there was a lot of internal strife within the nascent Muslim community. Abu Bakr had to deal with revolts and/or independent powers within Arabia immediately upon his ascension, and all three of his successors were assassinated. I don't know much about Umar, but Uthman's death was the product of dissent both in the provinces, particularly Egypt, and at home in Medina. Not exactly a case of harmonious co-operation between all Muslims. His successor, Ali, had to face off against the powerful governor of Syria, Mu'awiya, and was even opposed by Aisha, Muhammad's widow, which resulted in the First Fitna, or the first Muslim civil war. This culminated in Ali's assassination by people who were once on his side.

Behind all these conflicts was the inherent tension between different individuals with very different interests. There might be a unifying ideology, but these individuals still had clan and regional interests that cannot be removed so easily. Nor should we ignore obvious things like personal differences, naiviety and simple chance. Conflict between individuals is a fact of life and we should never see the past as an ideal world free from human vices. Though a bit dated, Hugh Kennedy's The Prophet and the Age of the Caliphates: The Islamic Near East from the 6th to the 11th Century is still a great introduction to this, as he expertly untangled the complex web of loyalties and alliances of the major players in early Islam.